Four-stroke spark-ignition engines have been developed to a very high level. Engines of the type used in Formula 1 racing cars are being produced that have an extraordinarily high power/cylinder-capacity ratio and an extraordinarily high power/weight ratio. However, due to the materials and techniques that are used to produce such engines and the low production quantities, they are also extraordinarily expensive. They also need much maintenance and have a short life. However, engines are also being produced in large quantities for the sports motorcycle market. They have a high power/cylinder-capacity ratio and a high power/weight ratio, even in standard tune, and due to mass-production techniques, they are relatively inexpensive. Compared to Formula 1 engines, they require less maintenance and have a long life. Although there are exceptions, such motorcycle engines are generally not much larger than 1000 cc in cylinder capacity. An example of such an engine is the Yamaha YZF@1OOOR Thunderace, an in-line four-cylinder cross-flow engine with a capacity of 1002 cc. There are five valves per cylinder (three inlet and two exhaust) operated by two overhead camshafts that are driven from a chain sprocket half-way along the crankshaft by a single timing chain extending from the crankshaft to the camshafts in a space between the middle two cylinders. In standard tune, this engine develops about 145 to 150 brake horsepower (108 to 112 kW). An example in the patent literature of such an arrangement is document JP61-197705A (Yamaha).

For car racing, but not in the Formula 1 league, and for sports cars, there is a need for engines having similar characteristics to a modern sports motorcycle engine such as the Yamahas YZF@lOOOR, but with a larger cylinder capacity, such as 2000 cc. An attempt has been made in the past to mount the cylinder banks of two such motorcycle engines on a common crankcase in a 90° V configuration to form a V8 engine. The cylinder banks are slightly staggered in the axial direction of the crankcase, and each of the four crank journals of the crankshaft is coupled to two of the connecting rods, one for each cylinder bank, side by side. This prior attempt appears not to have met with success. A problem that arises is how to

get the drive from the crankshaft to the camshafts in the two cylinder banks. Because of the desire that the sprockets on the camshafts be as small as possible so as not to take up much space, and the requirement that the crankshaft sprocket be half the diameter of the camshaft sprockets to achieve four-stroke operation, the standard motorcycle crankshaft is weak in the region of the driving sprocket for the timing chain (see Figure 9, reference 34), and space is limited. As will be described in more detail below, providing two spaced-apart driving sprockets (see Figure 10, references 34L, 34R) for the two timing chains in the V configuration weakens the crankshaft even more and takes up more space in the axial direction than is readily available. An example in the patent literature of such an arrangement is document JP58-128421A (Yamaha).

The present invention, or at least specific embodiments of it, is concerned with these problems.

In accordance with the present invention, the middle of the crankshaft is formed with a driving gear, and a plurality of idler units are provided, one for each bank. Each idler unit has a driven gear driven directly or indirectly by the crankshaft gear, and a chain-sprocket axially offset from the driven gear for driving the timing chain of the respective bank. The drive for the camshafts can therefore be taken off the crankshaft in a single general plane so as not to take up much space on the crankshaft in the axial direction of the crankshaft, and the idler units then provide the required staggering for the drives to the camshafts. Furthermore, four-stroke operation does not place any direct restriction on the size of the crankshaft gear as compared with the size of the camshaft sprockets, and so the crankshaft gear can be formed of a size such that it does not weaken the crankshaft (see Figure 11, reference 44).

The diameter of the crankshaft in the region of the crankshaft gear is preferably not substantially less than the diameter of the main journals, and indeed the diameter of the crankshaft gear is preferably substantially greater than the main journals. Preferably, two main journals are disposed immediately to either side of the crankshaft gear.

For simplicity, the gear ratio of the crankshaft to the idler units may be 1: 1, in which case the gear ratio of the idler units to the camshafts would need to be 2: 1 in order to achieve four-stroke operation. However, if the number of teeth on the camshaft sprockets permits, the gear ratio of the crankshaft to the idler units may greater than this, such as 4: 3 (or 3: 2), in which case the gear ratio of the idler units to the camshafts would need to be 3: 2 (or 4: 3) in order to achieve four-stroke operation. This has the benefit that there is not such a tight wrap of the timing chains around the idler sprockets.

Preferably, at least one of the idler gears is directly driven by the crankshaft gear, for example using spur or helical gearing rather than sprockets and chain.

In one embodiment of the invention (see Figures 5 & 6), each idler gear is directly driven by the crankshaft gear. However, considering a typical V configuration with the inlets inboard of the V and the exhausts outboard of the V, this has the effect that the camshaft (s) of one bank rotate in the opposite direction to the camshaft (s) of the other bank relative to the gas flow direction through the respective bank. In the case of a single camshaft per bank, the two camshafts would therefore need to be oppositely-handed. In the case of double camshafts per bank (inlet and exhaust) with symmetrical cam lobes, it may be possible that the inlet camshafts of the two banks could be identical and the exhaust camshafts of the two banks could be identical. However, in the case of, for example, the Yamaha engine mentioned above, the cam lobe for one of the inlet valves per cylinder is offset from the cam lobes for the other two inlet valves for that cylinder, and so in that case the two inlet camshafts would need to be oppositely- handed. Furthermore, when a timing chain tensioner is provided, it desirably acts on the lower- <BR> <BR> tension run of the timing chain (i. e. the run to the camshaft (s) ). Therefore, in the typical V configuration mentioned above, the two timing chain tensioners would need to be oppositely positioned relative to the gas flow direction through the respective bank.

In order to enable the two banks to be identical in all general respects including camshafts and chain tensioner positions, in another embodiment of the invention, the gear of one of the idler units is driven by the gear of the other, or another, of the idler units (see Figures 7 & 8). Accordingly, in the typical V configuration mentioned above, the camshaft (s) of the two banks rotate in the same direction relative to the gas flow direction through the respective bank, and the timing chain tensioners can be in identical positions relative to the gas flow direction through the respective bank.

At least in the V configuration mentioned above, the centre spacing between the gear and sprocket of each idler unit is preferably substantially equal to one half of the axial staggering of the cylinder banks.

Specific embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectioned end view of the cam drive of a conventional engine, sectioned on the line 1-1 shown in Figure 2, and with the outline of the engine shown in phantom lines;

Figure 2 is a schematic plan view of the crankshaft of the engine of Figure 1, with the crankcase shown in phantom lines; Figure 3 is a schematic sectioned end view of the cam drive of a modified form of the engine of Figures 1 and 2, sectioned on the line 3-3 shown in Figure 4, and with the outline of the engine shown in phantom lines; Figure 4 is a schematic plan view of the crankshaft of the engine of Figure 3, with the crankcase shown in phantom lines; Figure 5 is a schematic sectioned end view of the cam drive of an engine of a first embodiment of the invention, sectioned on the line 5-5 shown in Figure 6, and with the outline of the engine shown in phantom lines; Figure 6 is a schematic plan view of the crankshaft of the engine of Figure 5, with the crankcase shown in phantom lines; Figure 7 is a schematic sectioned end view of the cam drive of an engine of a second embodiment of the invention, sectioned on the line 7-7 shown in Figure 8, and with the outline of the engine shown in phantom lines; Figure 8 is a schematic plan view of the crankshaft of the engine of Figure 7, with the crankcase shown in phantom lines; Figure 9 is a schematic plan view of the crankshaft of the engine of Figures 1 and 2; Figure 10 is a schematic plan view of the crankshaft of the engine of Figures 3 and 4; and Figure 11 is a schematic plan view of the crankshaft of the engines of Figures 5 to 8.

In the drawings, transmission chains and the teeth of chain sprockets and are denoted, in side view, by diagonal cross-hatching, and the teeth of spur gears are denoted, in side view, by square cross-hatching.

Referring to Figures 1,2 and 9, a crankshaft 10 of a conventional four-cylinder, twin- overhead-camshaft four-stroke engine 12 has six main journals 14 that are supported in six main bearing blocks 16 formed in the engine's upper and lower crankcase halves 18,20. The crankshaft 10 has four crank journals 22, each joined to the adjacent main journals 14 by a pair of webs 24. In order from one end of the crankshaft 10, the angular dispositions of the four

crank journals 22 are 0°, 180°, 180° and 0°. Each of the four cylinders (not shown) in the cylinder block 26 has a respective piston (not shown) connected by a connecting rod (not shown) to a respective one of the four crank journals 22. A cylinder head 28 is mounted on top of the cylinder block 26 and has an exhaust camshaft 30 and an inlet camshaft 32 to operate the exhaust and inlet valves (not shown) of the engine 12. A chain-sprocket 34 is integrally formed in the crankshaft 10 half-way along its length, between the middle two main journals 14. The exhaust camshaft 30 has an exhaust chain-sprocket 36 half-way along its length, and the inlet camshaft 32 has an inlet chain-sprocket 38 half-way along its length. The exhaust and inlet sprockets 36,38 lie in the same plane as the crankshaft sprocket 34. The exhaust and inlet sprockets 36,38 each have twice as many teeth as the crankshaft sprocket 34. The three sprockets 34,36, 38 are coupled by a timing chain 40 extending through an opening 42 in the top of the upper crankcase half 18 and a cavity (not shown) in the cylinder block 26 between the middle two cylinders. In operation, the crankshaft 10 and camshafts 30,32 rotate counter- clockwise as viewed in Figure 1, and a chain tensioner 42 is provided on the lower-tension run of the timing chain 40 from the crankshaft sprocket 34 to the inlet camshaft sprocket 38.

The exhaust and inlet sprockets 36,38 are preferably as small as possible so that they do not take up too much room. However, the crankshaft sprocket 34, which has to be half the size of the exhaust and inlet sprockets 36,38 in order to obtain four-stroke operation in such a configuration of engine, is a weak point in the crankshaft 10, and so there is a limit on the minimum sizes of the sprockets 34,36, 38.

If it were desired to modify the engine of Figures 1 and 2 so as to have a pair of cylinder blocks 26L, 26R and cylinder heads 28L, 28R, generally identical to those shown in Figures 1 and 2, mounted in a 90° V formation on a modified upper crankcase half 18 and with a modified crankshaft 10, a configuration as shown in Figures 3,4 and 10 might be considered.

In Figures 3 and 4, the cylinder blocks 26L, 26R are staggered relative to each other in the axial direction of the crankshaft 10 by a distance B equal to the length of the big-end bearings that connect the lower ends of the connecting rods to the crank journals 22. Also, the length of each crank journal 22 is 2xB, i. e. they are twice the length of those in the engine of Figures 1 and 2.

Each crank journal 22 is connected to a pair of the connecting rods (not shown), one from the left bank and the other from the right bank. Two crankshaft sprockets 34L, 34R are formed half- way along the crankshaft 10, with a centre spacing of B, so that one crankshaft sprocket 34L lies in the same plane as the exhaust and inlet sprockets 36L, 38L of the left bank, and the other crankshaft sprocket 34R lies in the same plane as the exhaust and inlet sprockets 36R, 38R of the right bank. The left-bank sprockets 34L, 36L, 38L are coupled by one timing chain 40L, and the

right-bank sprockets 34R, 36R, 38R are coupled by another timing chain 40R. For ease of plumbing the fuel system and exhaust system, the inlet camshafts 32L, 32R are inboard of the V, and the exhaust camshafts 30L, 30R are outboard of the V. The chain tensioner 42L of the left bank acts on the run of the left timing chain 40L from the crankshaft sprocket 34L to the inlet sprocket 38L, as in the case of the engine of Figures 1 and 2. However, so that the chain tensioner 42R of the right bank can act on the lower-tension run of the right timing chain 40R, the cylinder block 26R has to be modified as compared with the engine of Figures 1 and 2 so that the right chain tensioner 42R acts on the run of the right timing chain 40R from the crankshaft sprocket 34R to the exhaust sprocket 36R, rather than to the inlet sprocket 38R. It will be appreciated that the exhaust and inlet camshafts 30R, 32R of the right bank rotate in the opposite direction as compared with the engine of Figures 1 and 2. However, that is of no consequence if the cam lobes are symmetrical.

It will be recalled that the crankshaft 10 of Figures 1,2 and 9 has a weak point in its centre due to the crankshaft sprocket 34. By comparing Figures 9 and 10, it will be appreciated that the crankshaft 10 of Figures 3,4 and 10 has an even weaker point in its centre due to the provision of two crankshaft sprockets 34L, 34R. Furthermore, by comparing Figures 2 and 4 and Figures 9 and 10, it will be seen that the length of the two middle main journals 14 and the corresponding bearing blocks 16 has to be reduced to make space for the two crankshaft sprockets 34L, 34R, thus causing even further weakness of the engine 10 of Figures 3,4 and 10.

The two embodiments of the invention that will now be described are concerned with reducing that weakness.

The engine 12 of the first embodiment of the invention is shown in Figures 5,6 and 11, and is similar to the engine 12 of Figures 3,4 and 10 except in the following respects.

Rather than having a pair of chain-sprockets 34L, 34R of small diameter in the centre of the crankshaft 10, the engine 12 of Figures 5,6 and 11 has a single spur gear 44 formed in the centre of the crankshaft 10. Two idler gear units 46L, 46R are disposed above and to either side of the crankshaft 10. Each idler gear unit 46L, 46R has a shaft 48 that is rotatably mounted at its ends in the middle two bearing blocks 16. Integrally formed at the middle of each shaft is a spur gear 50 having the same number of teeth as the crankshaft spur gear 44 and meshing with the crankshaft spur gear 44. To one side of each idler gear 50, an idler chain-sprocket 52 is integrally formed with the shaft 48. Each idler sprocket 52 has half the number of teeth as the camshaft sprockets 36L, 36R, 38L, 38R. For each idler gear unit 46L, 46R, the centre spacing of the gear 50 and sprocket 52 is l/2B. The sprocket 50 of the left idler unit 46L lies in the same

plane as the left camshaft sprockets 36L, 38L and is coupled to them by the left timing chain 40L. The sprocket 50 of the right idler unit 46R lies in the same plane as the right camshaft sprockets 36R, 38R and is coupled to them by the right timing chain 40R.

In the engine of Figures 5,6 and 11, the crankshaft gear 44 is of sufficiently large diameter that it does not cause any waisting of the crankshaft 10 as compared with the diameter of the main journals 14, as can be seen by comparing Figures 10 and 11. Furthermore, the crankshaft gear 44 is sufficiently thin that it does not require the length of the two middle main journals 14 and the corresponding bearing blocks 16 to be reduced as compared with the other main journals 14 and bearing blocks 16.

As mentioned above in connection with Figures 3 and 4, and true also of the engine 12 of Figures 5 and 6, the exhaust and inlet camshafts 30R, 32R of the right bank rotate in the opposite direction as compared with the engine of Figures 1 and 2. Although, this is of no consequence if the cam lobes are symmetrical, some engines do not have symmetrical cam lobes. For example, in the case of an engine with three inlet valves per cylinder, the cam lobe for actuating the middle inlet valve may be angularly offset relative to the cam lobes for actuating the other two inlet valves. Furthermore, as mentioned above in connection with Figures 3 and 4, and true also of the engine 12 of Figures 5 and 6, the position of the chain tensioner 42R for the right bank needs to be modified as compared with the position of the chain tensioner 42L for the left bank or the chain tensioner 42 of the engine 12 of Figures 1,2 and 9.

In the case where the engine is to be built from off-the-shelf cylinder block and cylinder head assemblies, it would be desirable if it were not necessary to have to modify the position of the chain tensioner 42 in one of the cylinder block assemblies and to manufacture an oppositely- handed camshaft 32 for one of the cylinder head assemblies. The second embodiment of the invention obviates the need for this.

The engine 12 of the second embodiment of the invention is shown in Figures 7,8 and 11 and is similar to the engine 12 of Figures 5,6 and 11 and has an identical crankshaft 12, but differs in the following respects.

In the engine of Figures 7,8 and 11, the positions of the idler gear units 46L, 46R are modified. As before, the spur gear 50 of the left gear unit 46L meshes with the crankshaft gear 44. However, the spur gear 50 of the right gear unit 46R meshes with the spur gear 50 of the left gear unit 46L, but does not mesh with the crankshaft gear 44. Accordingly, the camshafts 30L, 30R, 32L, 32R rotate in the same directions as the camshafts 30,32 of the engine 12 of Figures 1,2 and 9 relative to the cross-flow directions of the respective banks, and each chain

tensioner 42L, 42R needs to act on the run of the timing chain 40L, 40R from the driving sprocket 52 to the inlet sprocket 38L, 38R, as in the case of the engine 12 of Figures 1,2 and 9.

It will be noted that, in view of the asymmetrical positioning of the idler gear units 46L, 46R, the right timing chain 40R is shorter than the left timing chain 40L. Preferably, the positioning of the idler gear units 46L, 46R is such than, when both chain tensioners 42L, 42R are in their mid positions of adjustment, the difference in length of the two timing chains 40L, 40R equates to an integer number of chain links, for example two links.

Many modifications and developments may be made to the embodiments of the invention described above. For example, helical, rather than spur, gearing may be used between the crankshaft 10 and the idler gear units 46L, 46R. In addition to its application to V engines, the invention may also be applied to, for example, axially-opposed engines and X engines.

Furthermore, the invention may be applied to engines having two, six or a higher even number of cylinders per bank.

It should be noted that the embodiments of the invention have been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.